The present invention relates to neutron backscatter techniques for monitoring fluid levels such as water or oil levels in a vessel. These techniques are generally well known, and because they are non-intrusive, are particularly useful in certain situations where hydrogen-bearing liquid levels need to be monitored. Examples of prior art patents relating to this technology include U.S. Pat. No. 4,038,548 (Charlton, issued July 26, 1977), and U.S. Pat. No. 4,216,376 (Griffin et al, issued Aug. 5, 1980). An example of density measurement with neutron technics is shown in U.S. Pat. No. 4,582,991 (Leonardi-Cattolica et al., issued Apr. 15, 1986).
The principles involved in using neutrons to detect the presence of hydrogen are well known, for example in well logging devices and in certain types of moisture gauges. However, comparatively little attention has been paid to using these principles for solving fluid level measurement problems. In an industry like the petrochemical industry, where raw and processed materials nearly always contain substantial amounts of hydrogen, level measurement instruments employing neutrons could have wide application.
U.S. Pat. No. 4,038,548 (above) discloses a device from which portable level detectors and fixed level monitors could be made based on neutron scattering and detection principles, to indicate the presence or absence of a hydrogen bearing liquid opposite the detector. The main components of such a device (a "neutron backscatter gauge") would be a neutron emitting radioisotope, a neutron detector, and a ratemeter. Such a gauge would be a good detector for hydrogen but much less sensitive to most other materials. This would allow level measurements of hydrogen bearing materials, such as hydrocarbons, to be made through the walls of metal vessels. Measurements could be conveniently made through steel walls a few inches thick.
For detecting liquid at specific, discrete levels, neutron backscatter gauges have been used in a wide variety of level measurement applications, such as those encountered in the petrochemical industry. In a number of cases, the neutron techniques have proven to be superior to conventional level measurement methods, including gamma ray methods.
In a typical discrete level measurement application, the principal components of a neutron backscatter gauge (the neutron source, the neutron detector and the ratemeter) are arranged with the source mounted next to the neutron detector, and together they are placed near the exterior of the vessel wall. The source must emit energetic neutrons. .sup.252 Cf (californium) sources have typically been used in this work but other sources such as .sup.241 Am/Be (americium/beryllium) are also suitable. The average neutron energies for the .sup.252 Cf and .sup.241 Am/Be sources are on the order of a few million electron volts (MeV). Neutron emission rates in the range 10.sup.4 -10.sup.6 neutrons/sec are adequate for most applications. These rates are so low that the sources are easily handled in a safe manner.
.sup.3 He detectors are used because of their high thermal neutron detention efficiency. The detector consists of a sealed tube filled with gas, .sup.3 He being the main component. Energetic, charged nuclear particles are generated inside the detector by the neutrons which enter the detector and react with the .sup.3 He. The nuclear reaction is EQU .sup.3 He+n.fwdarw..sup.3 H+p+0.765 MeV (1)
The energy released by the reaction is shared between the tritium nucleus and the proton. The detector tube also contains a central electrode on which a voltage is applied to maintain a voltage gradient between the central electrode and the tube wall. When such an energetic, charged nuclear particle passes through the gas in the tube, some of the gas molecules are ionized. The ions and electrons are accelerated by the applied electric field, thereby producing a current pulse. The number of current pulses generated per unit time (pulse rate) is thus related to the number of neutrons entering the detector per unit time. The current pulses are counted by the ratemeter. The output of the ratemeter is a signal which is proportional to the pulse rate.
The cross section for reaction (1) is a strong function (approximately .about.E.sup.-1/2) of the kinetic energy (E) of the neutron. The cross section is less than 1 barn for neutron energies above a few tenths of an MeV. The detector is thus insenstive to neutrons coming directly from the source because the reaction cross section for these energetic neutrons (E&gt;1 MeV) is so small. This allows the source to be placed next to the detector in a backscatter gauge. On the other hand, .sup.3 He detectors have very high detection efficiencies for low energy neutrons because the corresponding reaction cross section is very large. The thermal neutron cross section (neutron energy equal to 0.025 eV) is 5330 barns. .sup.3 He detectors are available which have efficiencies approaching 100 percent for thermal neutrons.
The strong dependence of detector sensitivity on neutron energy makes the backscatter gauges very sensitive to the presence of materials which are good moderators, i.e., those that are efficient in reducing the energies of neutrons to the thermal range where the detectors are efficient. When the backscatter gauge is placed in the immediate vicinity of a good moderator, the number of low energy neutrons reaching the detector increases substantially and the change in the number of current pulses produced per unit time is easily measurable.
Hydrogen is the most effective moderator, so materials which contain substantial amounts of hydrogen are effective moderators. Hydrogen is effective for two reasons:
1. The elastic scattering cross section for hydrogen is relatively large, and
2. In most collisions between a neutron and a hydrogen nucleus, a substantial fraction (on the average one-half) of the energy of the neutron is transferred to the hydrogen nucleus. Roughly 20 collisions with hydrogen are required to reduce the energy of a 1 MeV neutron to the thermal range. A much larger number of collisions is required to achieve this result with all but the very lightest of the other elements.
A neutron backscatter gauge is thus very sensitive to the presence of hydrogen bearing materials but relatively insensitive to the presence of non-hydrogen bearing materials commonly encountered in petrochemical plants. Therefore, a neutron backscatter gauge can be viewed as a "hydrogen detector" in many level measurement applications. In a typical application, a vessel wall made of a non-hydrogen bearing material like steel separates the neutron backscatter gauge from a hydrogen bearing material whose level is to be measured. If the separation is not too great, the presence of the vessel wall does not prevent level measurements from being made. By moving the gauge along the vessel vertically, a water level can be determined through a one inch steel plate in 2 or 3 minutes to an accuracy of approximately an inch, without using calibration procedures.
In most cases, level measurements can be made from the exterior of a vessel. However, if material which contains a substantial amount of hydrogen surrounds the vessel, it may be difficult or impossible to make the desired measurement externally. A vessel with a water or oil jacket, for example, might require that the neutron backscatter gauge be mounted in a well within the vessel.
A neutron backscatter gauge is only sensitive to material located within inches of the source and detector. Therefore, the geometry in which the source and detector are close to or coincident with one another, which results in the detection of low energy "backscattered" neutrons, is generally employed in level measurements applications. However, transmission measurements, in which the source and detector are separated to allow a moderator to be interposed, are useful for some applications. Also, the neutron backscatter gauge can be used to determine the level of any type of interface, not just liquid-gas interfaces such as water-air or oil-air.
Very low intensity neutron sources can be used in most level measurement applications. Since heavy shielding for personnel protection is not required, it is also possible to build a portable backscatter gauge weighing only a few pounds. The principles which apply to measurements made with the portable backscatter gauge are the same as those which apply to fixed level monitors. One such portable gauge consisted of a .sup.252 Cf source, a .sup.3 He detector (4 atmosphere pressure, 1 inch diameter, 6 inch active length), a 36 inch wand to which the source and detector were attached, and a battery powered combination ratemeter/scaler. The source and detector were mounted on one end of the wand as a means of keeping the source away from the operator when the instrument was in use. For a .sup.252 Cf source with an emission rate of 2.3.times.10.sup.5 neutrons/sec (0.1 micrograms of .sup.252 Cf), the combined gamma ray/neutron exposure rate measured at the end of the wand away from the source was less than 0.25 mrem/hr. That is more than a factor of ten below the maximum permissible occupational exposure level of 2.5 mrem/hr for a 40 hour work week. The exposure rate at a distance of 1 foot from the source was approximately 2 mrem/hr. An 8 inch diameter boron impreganted polyethylene bioshield provided personnel protection when the instrument was being transported or stored. The exposure rate measured at the surface of the bioshield was less than 9 mrem/hr for the 0.1 microgram .sup.252 Cf source. It was less than 2 mrem/hr 6 inches from the surface of the bioshield.
Such a portable neutron backscatter gauge can be a valuable tool for measuring levels. It is easy to use and is often the most convenient means of (1) measuring levels in vessels which have no level sensing devices and (2) checking or calibrating level sensors, such as differential pressure cells and gamma ray level gauges, after installation. The gauge has also been used to detect two phase flow in pipes and to determine if pipes were empty before cutting into them.
However, both the fixed level type monitor and the portable type level detector also have significant disadvantages. Perhaps one of the greatest of these is their being limited to detecting essentially a single level, i.e., the presence or absence of a neutron moderator (e.g., hydrogen) at a given location. Of much greater utility in many applications would be a neutron-moderation based fluid level detector which would measure and indicate the fluid level over a wide range, well beyond just those levels immediately adjacent the relatively short detector.
It is easy to see why prior art neutron backscatter level detectors detect the liquid level essentially at only discrete places or over only short intervals, typically where the detector is located or the source is located. At that level, when the fluid/gas interface passes, a sharp transition occurs in the count rate for the moderated neutrons, thereby "detecting" the liquid level. At levels in between, however, no sharp transition, and thus no meaningful indication, is provided. Therefore, if the level is to be detected throughout a range, or at a lot of different levels, it is then customary to employ a correspondingly large number of sources and/or detectors.
A need therefore remains for an improved neutron backscatter fluid level detector which can provide an essentially continuous indication of the fluid level over an extended range without requiring an unreasonably large number of neutron sources and/or detectors. Preferably, such a fluid level detector should be not only uncomplicated in its design and implementation, but also inexpensive, versatile, and capable of providing reliable fluid level detection and indication in the widest possible range of applications.